Female very resistant to weight loss

I need some help here as I am at my wits end. My wife is struggling with her weight for the last few years. In the past she has been fit and was an aerobics instructor and swam on a regular basis. We have two children and she has spent the majority of her time and effort on them. For the last two year she has been trying to loose weight and in fact continues to gain. I have a degree in Exercise Physiology and have trained many men and women over the years and am an avid fitness freak. We have tried everything. Changing food intake, interval training, stims, thermo, long slow distance, straight lifting, combination of pretty much everything I can think of let alone borrow from this forum. I need some help on a thought. Here is what I think is the problem. Let me know if you think I am on crack. Probably five years back she had a lump on her thyroid. The docs opened her up and took it and half of her thyroid with it. Every year on her physical she gets it tested and they say it is low but still in the range. Am I crazy thinking that in the range may be the issue here? She cooks for me and so I know what she eats. So short of her sneaking Snickers at work I am 100% it is not diet. We tried Thyroid Energy from Now but she feels absolutely the same. At this point she is ready to try and listen to any advice. Thanks for the insights. Cheers

Thank you to all who replied. We generally stayed on a specific diet (for lack of a better word) for three months. We tried a 5 day work out approach and her fatigue level was crazy. She basically was going to bed at 7 or 8 every night and could not get up in the am. Currently she is walking every morning and working out 3 times a week. This seems to fit her lifestyle and is manageable energy wise. We recently ended a three month CKD to which she gained 2 lbs. She booked an appointment to go back and discuss this with her current GP. She has been great and hope fully will assist in sending in the right direction with some blood tests.

I have Hashimoto's.. and have to take thyroid hormone. My doc has me on enough to keep me in the high range, keeping my metabolism a bit boosted. When my thyroid dropped I gained weight that was very hard to get rid of. When she bumped me back up into the high end of normal, I got rid of it. I think a second opinion or more questions for her doctor is definitely worth a shot.